Neutron-induced Fission Of 232Th Research Articles

Tests of ionization chambers for future photofission experiments

A recent set-up of a multi-cathode Frisch-grid ionization chamber and experimental results of a proof-of-principle experiment are presented. Preliminary data on the obtained polar angular and mass distributions as well as total kinetic energy of fission fragments produced in neutron-induced fission of 232Th and 238U are discussed that show that the chamber has been operated successfully. For the additional measurement of the azimuthal angular distribution, a design of segmented anodes has been tested. Preliminary data do not allow a suffcient reconstruction of the azimuthal angle.

Measurement of fission products yields in the quasi-mono-energetic neutron-induced fission of 232Th

The cumulative yields of various fission products in the 232Th(n, f) reaction at average neutron energies of 5.42, 7.75, 9.35 and 12.53 MeV have been determined by using an off-line γ-ray spectrometric technique. The neutron beam was produced from the 7Li(p, n) reaction by using the proton energies of 7.8, 12, 16 and 20 MeV. The mass chain yields were obtained from the cumulative fission yields by using the charge distribution correction of medium energy fission. The fine structure in the mass yield distribution was interpreted from the point of nuclear structure effect. On the other hand, the higher yield around mass number 133–134 and 143–144 as well as their complementary products were explained based on the standard I and standard II asymmetric mode of fission. From the mass yield data, the average value of light mass (〈AL〉), heavy mass (〈AH〉), the average number of neutrons (〈ν〉) and the peak-to-valley (P/V) ratios at different neutron energies of present work and literature data were obtained in the 232Th(n, f) reaction. The different parameters of the mass yield distribution in the 232Th(n, f) reaction were compared with the similar data in the 232Th(γ, f) reaction at comparable excitation energy and a surprising difference was observed.

Anisotropy of the fission fragments from neutron-induced fission in the intermediate energy range of 1–200 MeV

Angular distributions of fission fragments from the neutron-induced fission of 232Th, 235U, and 238U have been measured in the energy range 1–200 MeV at the neutron time-of-flight (TOF) spectrometer GNEIS using position sensitive multiwire proportional counters as fission fragment detector. A short description of the experimental equipment and measurement procedure is given. The anisotropy of fission fragments deduced from the data on measured angular distributions is presented in comparison with experimental data of other authors.

Measurement and Covariance Analysis of Reaction Cross Sections for 58Ni(n,p)58Co Relative to Cross Section for Formation of 97Zr Fission Product in Neutron-Induced Fission of 232Th and 238U at Effective Neutron Energies En = 5.89, 10.11, and 15.87 MeV

The 58Ni(n,p)58Co reaction cross sections have been measured relative to two monitors: the cross sections for the formation of the 97Zr fission product in neutron-induced fission of (a) 232Th and of (b) 238U. It is demonstrated how to generate and combine covariance matrices (using partial uncertainties and microcorrelations) in relative measurements at various stages like efficiency calibration of the high-purity germanium detector, using the ratio of 58Ni(n,p)58Co reaction cross section relative to monitor cross section, and in the process of normalization. We further illustrate the weighted averaging of equivalent data as applicable in relative measurements. We provide the necessary data and the corresponding table of partial uncertainties as required for compilation in the EXchange-FORmat (EXFOR) database. This helps, in principle, anyone to generate and verify the steps in the calculation of the covariance matrices in the present work. We believe that it is important for all nuclear experimental scientists to incorporate a detailed data reduction procedure, reduced data, and partial uncertainties in their publications, to the extent possible, which will be very useful in EXFOR compilation.

Mass distribution in the quasi-mono-energetic neutron-induced fission of 232Th

The cumulative yields of various fission products in 232Th(n, f) with average neutron energies of 6.35, 8.53 and 10.09 MeV have been determined using an off-line \(\gamma\)-ray spectrometric technique. The neutron beam was produced from the 7Li(p, n) reaction. From the cumulative fission yields, the mass chain yields were obtained by using charge distribution correction of medium energy. The peak-to-valley (\(P/V\)) ratio, the average value of light mass (\(\langle A_{L}\rangle\)), heavy mass (\(\langle A_{H} \rangle\)) and the average number of neutrons (\(\langle\nu \rangle\)) at the three different neutron energies of the present work and at other energies from the literature in the 232Th(n, f) reaction were obtained from the mass yield data. The present and the existing literature data in the 232Th(n, f) reaction at various excitation energies were compared with similar data in the 238U(n, f) reaction. The fine structure in the mass yield distribution was interpreted from the point of nuclear structure effect such as shell closure proximity and even-odd effect. The role of standard I and standard II asymmetric mode of fission was discussed. The different types of mass-yield distributions between 232Th(n, f) and 238U(n, f) reactions were explained from different types of the potential energy between the two fissioning systems. The role of excitation energy was also investigated.

Fission Fragment Angular Distribution of 232Th(n,f) at the CERN n_TOF Facility

The angular distribution of fragments emitted in neutron-induced fission of 232Th was measured in the white spectrum neutron beam at the n_TOF facility at CERN. A reaction chamber based on Parallel Plate Avalanche Counters (PPAC) was used, where the detectors and the targets have been tilted 45∘ with respect to the neutron beam direction in order to cover the full angular range of the fission fragments. A GEANT4 simulation has been developed to study the setup efficiency. The data analysis and the preliminary results obtained for the 232Th(n,f) between fission threshold and 100 MeV are presented here.

Fission Product Yield in the Neutron-Induced Fission of 232Th with Average Energies of 5.42, 7.75, and 10.09 MeV

The yields of various fission products in the neutron-induced fission of 232Th have been determined a using recoil catcher and off-line gamma-ray spectrometric technique with flux-averaged energies of 5.42, 7.75, and 10.09 MeV. The neutrons were generated using the 7Li(p,n) reaction at the BARC-TIFR [Bhabha Atomic Research Centre–Tata Institute of Fundamental Research] Pelletron facility, Mumbai, India. The fission product–yield data in the 10.09-MeV neutron-induced fission of 232Th are determined for the first time. The yields of the different fission products in the neutron-induced fission of 232Th with flux-averaged energies of 5.42 and 7.75 MeV from the present work have been compared with similar data of comparable neutron energy from the literature and are found to be in good agreement. The effect of nuclear structure on fission product yields as a function of neutron energy has been examined.

Measurement of Fragment Mass Yields in Neutron-Induced Fission of 232Th and 238U at 33, 45, and 60 MeV

Incineration of spent nuclear fuel in Accelerator Driven Systems requires nuclear data on neutron-induced fission of actinides at energies above 20 MeV - the upper limit of conventional evaluated nuclear data files. Over the past years, a significant effort has been devoted to measurements of neutron-induced fission cross-sections at intermediate energies but there is a lack of experimental data on fission yields. Here we describe recent measurements of pre-neutron emission fragment mass distributions from intermediate energy neutron-induced fission of (232)Th and (238)U. The measurements have been carried out at the neutron beam of the Louvain-la-Neuve cyclotron facility CYCLONE. Quasi-monoenergetic neutrons with peak energies at 32.8, 45.3 and 59.9 MeV were produced by protons impinging on a thin lithium target. A multi-section Frisch-gridded ionization chamber was used as a fission fragment detector. The measurement results are compared with available experimental data as well as with predictions of the multi-modal random neck-rupture model implemented into the TALYS code.

Fission fragment velocities, masses, and energies from 232Th(n, f) in the resonance-energy region

Fragment velocities, masses, and energies from neutron-induced fission of 232Th were measured at various neutron energies in the subthreshold-resonance region around E n ≅ 1.6 MeV and above threshold with good neutron-energy resolution. These quantities were obtained by three different methods using two different types of detectors, namely parallel plate avalanche counters for time of flight, and surface-barrier detectors for time of flight and pulse height measurements. Comparison of the different results and a careful error analysis clearly exhibit the limitation in resolution due to methodical uncertainties. Within these limits, none of the measured quantities shows dependencies on neutron energy.

Fine structure of mass and charge distribution in low energy fission

Studies of finer details in mass and charge distribution fission leads to a better understanding of the fission process. Experimental determination of independent and cumulative yields using radiochemical techniques as well as mass spectrometers and fission product recoil separators form the basis of such studies. It has been established that closed shells as well as an even number of nucleons influence both mass and charge distributions. The magnitudes of these effects may be estimated from existing experimental yield data and various fission models. Using our measurements of several fission yields and those existing in the literature we have calculated even-odd proton and neutron effects for various low energy fissioning systems. Where enough data existed, direct calculations were made, whereas for other cases the Zp-model of WAHL has been used. It is found that the even-odd proton effect is well established and pronounced in thermal neutron fission of235U and233U. Lesser effects were found for reactor neutron induced fission of232Th, thermal neutron fission of239Pu and spontaneous fission of245Cm and249Cf. No effect seems to exist in the thermal neutron fission of241Pu and the spontaneous fission of252Cf. The even-odd neutron effect is found to be much lower than the corresponding proton effect in235U and233U fissions and is nonexistent in the rest of the fissioning systems.

Angular distributions of fragments in the fission of thorium induced by neutrons in the range 12–18 MeV

The angular distributions of fragments from neutron-induced fission of 232Th have been measured by means of glass detectors in the range 12.2 MeV ≦ E n ≦ 18.3 MeV. The behaviour of the angular anisotropy is analysed and the contribution of the (n, 2n′f) process is discussed.

Angular distributions and cross sections of fragments from neutron-induced fission of 232Th

Fission fragment angular distributions and cross sections were measured for the fission of 232Th induced by mono-energetic neutrons of energies 1.20±0.02, 1.40±0.02 and 1.60±0.02 MeV. The fission fragments are detected with approximately 2π geometry with a polycarbonate resin detector. The fission cross section and angular distribution are both changing markedly with neutron energy. Information on the single-particle levels in the transition nucleus 232Th is derived by fitting the experimental data with a Hauser-Feshbach type calculation which includes, in addition to the fission channels, the open neutron and gamma-ray decay channels taking into account level-width fluctuations.